1. Field of the Invention
This invention relates to a method of manufacturing an optical disk.
2. Background of the Invention
An after-recording type of optical disk having a structure as shown in FIG. 1 is known in the art. More specifically, the optical disk, as shown in FIG. 1, comprises a substrate 11, a data recording layer 12 formed on the substrate 11, and a protective board 15. The protective board 15 is bonded through a spacer 13 to the substrate 11 in such a manner that a space 14 is formed between the protective board 15 and the data recording layer 12.
Part (a) of FIG. 2 is an enlarged sectional view showing a part of FIG. 1 which is defined by the circle X in FIG. 1. Preaddress pits 111 and a guide groove 112 are coaxially juxtaposed on the transparent substrate 11. The data recording layer 12 which contains, for instance, organic pigment is formed over the preaddress pits 111 and the guide groove 112. Part (b) of FIG. 2 is a front view of the substrate 11 as viewed from the laser beam application side, corresponding to part (a) of FIG. 2. As is apparent from FIG. 2, a preaddress pit region PA and a guide groove region PG are formed on the substrate 11 in advance. A laser beam is then used to make holes in the data recording layer 12 on the guide groove region PG so that data are additionally recorded in the form of pits 113.
In general, in the substrate of this after-recording type at optical disk, the depth of the preaddress pits 111 is set to .lambda./4n (where .lambda. is the wavelength of a laser beam used, and n is the refractive index of the substrate 11) so as to maximize the contrast of the laser beam reflected therefrom. The depth of the guide groove 112 is set to .lambda./8n so as to maximize a tracking signal.
The above-described substrate having the guide groove and the preaddress pits is produced on a large scale as follows. First, a photo resist layer is formed on a circular glass plate which is a recording original board. Then the photo resist layer thus formed is selectively exposed to a laser beam and is developed to form a mother die. The mother die thus formed is utilized to form a metal stamper of Ni or the like. The metal stamper is used for a mass production of the substrate of PMMA or the like.
It is essential that the substrate thus formed is accurate in configuration. More specifically, in the substrate, the bottoms of the preaddress pits and the guide groove should not be rounded. For this purpose, in a method of forming the mother die for the stamper, as shown in FIG. 3, a first photo resist layer 2 and a second photo resist layer 3 are formed on the glass plate 1 by coating the glass plate 1 with two kinds of photo resist of different spectral sensitivity from each other, without forming an intermediate layer therebetween. Then, the photo resist layers are selectively exposed to a laser beam. In the method, the solvents of the two kinds of photo resist are different. Therefore, it is an absolute condition that the solvent of the second photo resist layer 3 will never affect the first photo resist layer 2. Accordingly, in the method, the range of selection of the photo resist is greatly limited.
This difficulty has been eliminated by a method disclosed by Japanese Patent Application No. 23010/1984. In the method, as shown in FIG. 4, an intermediate layer 4 of nonorganic material such as silicon oxide which will not be decomposed by the solvent is provided between the first and second photo resist layers 2 and 3.
In the method, the intermediate layer 4 of silicon oxide is an SiO or SiO.sub.2 film. Therefore the formation of the intermediate layer 4 needs a large, expensive device such as a vacuum deposition device or a sputtering device.